Transition Metals

State the meaning of the term transition metal complex. (1 mark)

Central metal ion surrounded by ligands

Explain the meaning of the chelate effect. (2 marks)

• When bidentate/multidentate ligands replace monodentate ligands to form a more stable complex

• Because there is an increase in entropy/positive entropy change/disorder or more particles formed (so ∆G is negative and ∆H is approximately 0)

Explain why complexes formed from transition metal ions are coloured. (3 marks)

• The ligands cause the energy levels of the d orbitals to split

• Some wavelengths of visible light are absorbed to promote d electrons from ground state to excited state

• The light that is not absorbed is transmitted to give the substance colour

Describe how a calibration graph is produced and used to find the concentration of the iron(III) complex. (3 marks)

• measure absorbance for (a range of) known concentrations

• plot graph absorbance v concentration

• read value of concentration for the measured absorbance from this graph

During the experiment the concentration of cisplatin is measured at one-minute intervals. Explain how graphical methods can be used to process the measured results, to confirm that the reaction is first order. (3 marks)

• plot concentration (y-axis) against time (x-axis) and take tangents / (calculate the) gradients (to calculate rates)

• Plot rate/gradients against conc

• straight line through origin / directly proportional confirms first order

Some complexes containing transition metal ions are coloured.

• Explain why some complexes containing transition metal ions are coloured

• List the factors that affect the colour

• Describe how colorimetry can be used to determine the concentration of a coloured complex (6 marks)

Stage 1 absorption of light

• d orbitals have different energy/d orbital (energies) are split

• Electrons move to higher (energy) (d) orbitals/electrons move to excited state

• Absorb visible/white light

• Colour seen is that from complementary colours/colours transmitted/reflected/not absorbed

Stage 2 reasons for different colours

• The metal

• The oxidation state (of the metal)/charge of metal (ion)

• The ligand(s)

• The co-ordination number/shape

Stage 3 Colorimetry

• Measure the absorbance for a range of (known) concentrations

• Plot graph of absorbance v concentration/calibration curve (of absorbance v concentration)

• Measure absorbance of the coloured complex and find concentration from graph

Explain why aqueous solutions containing [CuCl₄] ^2– ions are yellow. (2 marks)

• visible/white light absorbed (and (d) electrons excited)

• only yellow light transmitted/reflected

A student determines the concentration of ethanedioate ions in an acidified solution by titration with potassium manganate(VII) solution.

2 MnO4^– + 5 C₂O₄ ^2– + 16 H⁺ → 2 Mn²⁺ + 10 CO₂ + 8 H₂O

The mixture is warmed before the addition of potassium manganate(VII) solution because the reaction is slow at first. When more potassium manganate(VII) solution is added, the mixture goes colourless quickly due to the presence of an autocatalyst. Explain the meaning of the term autocatalyst. Explain, using equations where appropriate, why the reaction is slow at first and then goes quickly. (6 marks)

• autocatalysis: product of the reaction catalyses the reaction

• slow: negative ions repel / ions of same charge repel

• high E_a

• attraction between oppositely charged ions / negative reactant ion(s) and positive catalyst / Mn2+ / Mn3+

• 4 Mn²⁺ + MnO₄^– + 8 H⁺ → 5 Mn³⁺ + 4 H₂O

• 2 Mn³⁺ + C₂O₄^2– → 2 Mn²⁺ + 2 CO₂

Transition metal complexes have different shapes and many show isomerism. Describe the different shapes of complexes and show how they lead to different types of isomerism. Use examples of complexes of cobalt(II) and platinum(II). You should draw the structures of the examples chosen. (6 marks)

Stage 1: shapes of complexes

• octahedral or 6 co-ordinate diagram

• tetrahedral or square planar or 4 co-ordinate diagram

Stage 2: cis/ trans isomerism (or E-Z or geometric)

• cis/trans isomerism in either square planar and/or octahedral complexes 2

• Diagrams showing cis and trans isomerism in a square planar complex

• Diagrams showing cis and trans isomerism in both isomers of octahedral complexes eg draw cis and trans M(H₂O)₄(OH)₂ or [M(NH₃)₄(H₂O)₂]²⁺

Stage 3: optical isomerism

• optical isomerism / non superimposable mirror images in octahedral complexes

• occurs with a specific bidentate ligands eg.C₂O₄ ^2- or NH₂CH₂CH₂NH₂

• 3c draw both optical isomers of eg [M(NH₂CH₂CH₂NH₂)₃ ] ²⁺

Solution A contains the compound [Cu(H₂O)₆]Cl₂

State the type of bonding between the oxygen and hydrogen in this compound. (1 mark)

State why the chloride ions in this compound are not considered to be ligands (1 mark)

• Covalent

• Cl^- does not form a coordinate bond to Cu²⁺ as it does not donate a lone pair

Explain why colorimetry cannot be used to determine the concentration of solutions containing [CuCl₂]⁻ In your answer refer to the electron configuration of the metal ion. (2 marks)

• 3d10 or has full 3d (sub) shell/orbital

• It is colourless/cannot absorb frequencies of visible light

When the complex ion [Cu(NH3)4(H2O)2] 2+ reacts with 1,2-diaminoethane, the ammonia molecules but not the water molecules are replaced.

Write an equation for this reaction. Suggest why the enthalpy change for the reaction is approximately zero. (3 marks)

• [Cu(NH3)₄(H2O)₂] ²⁺ + 2H₂NCH₂CH₂NH₂ [Cu(H₂NCH₂CH₂NH₂)₂(H₂O)₂] ²⁺ + 4NH₃

• Cu–N bonds formed have similar enthalpy / energy to Cu–N bonds broken

• And the same number of bonds broken and made

Describe how a student could distinguish between aqueous solutions of potassium nitrate, KNO₃, and potassium sulfate, K₂SO₄, using one simple test-tube reaction.

• BaCl₂

• Observation with KNO₃(aq) - no (visible) change (nvc)

• Observation with K₂SO₄(aq) - white precipitate / white solid

Describe how a student could distinguish between aqueous solutions of magnesium chloride, MgCl₂, and aluminium chloride, AlCl₃, using one simple test-tube reaction. (3 marks)

• NaOH / sodium hydroxide

• white precipitate - but AlCl₃ dissolves in excess NaOH while MgCl₂ stays insoluble

Discuss the role of iron as a heterogeneous catalyst in the Haber process. 3 H2 + N2 ⇌ 2 NH3

Your answer should include:

• the meaning of the term heterogeneous catalyst

• how iron acts as a heterogeneous catalyst

• the factors that affect the efficiency and lifetime of the catalyst.

(6 marks)

Stage 1

• Heterogeneous means in a different phase/state from reactants

• Catalyst speeds up reaction and is left unchanged OR lowers the activation energy for the reaction

Stage 2

• Hydrogen and nitrogen/reactants adsorb onto the surface/ active sites of the iron

• Bonds weaken/reaction takes place

• Products desorb/leave from the surface (of the iron)

Stage 3

• Large surface area (of iron) by using powder or small pellets or support medium/mesh

• Catalyst poisoned / sulfur poisons or binds to the catalyst

• Active sites blocked

Explain why an aqueous solution containing [Fe(H₂O)₆] ³⁺ ions has a lower pH than an aqueous solution containing [Fe(H₂O)₆] ²⁺ ions. (3 marks)

• Fe3+ is smaller (than Fe2+) OR Fe3+ has a greater charge OR Fe3+ has a greater charge density OR Fe3+ has a greater charge to size ratio

• Fe3+ ions are more polarising OR Fe3+ ions polarise water molecules more

• So more O-H bonds (in the water ligands) break OR more H+ ions released OR weaken O-H bonds in ligands more (in the Fe3+ solution)

Explain, with the use of an equation, why a solution containing [Al(H₂O)₆]³⁺ has a pH ˂7. (3 marks)

• [Al(H₂O)₆] ³⁺ + H₂O ⇌ [Al(H₂O)₅(OH)] ²⁺ + H₃O⁺ OR [Al(H₂O)₆] ³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺

• Al 3+ has a small size and high charge OR has a high charge density

• Weakens the OH bond (in water) releasing H+ ions

Ethanedioate ions react with aqueous iron(III) ions in a ligand substitution reaction. Write an equation for this reaction. Suggest why the value of the enthalpy change for this reaction is close to zero. (2 marks)

• [Fe(H₂O)₆] ³⁺ + 3C₂O₄ ^2– —> [Fe(C₂O₄)₃] ^3– + 6H₂O

• There are 6 Fe –O bonds broken and then made / same number and type of bond being broken and made.

Ethanedioate ions are poisonous because they react with iron ions in the body. Ethanedioate ions are present in foods such as broccoli and spinach. Suggest one reason why people who eat these foods do not suffer from poisoning. (1 mark)

The ethanedioic acid is only present in small quantities/low concentration in these foods.

Describe a test you could carry out in a test tube to distinguish between sodium oxide and potassium oxide

• react with water

• measure pH

• sodium hydroxide higher pH

• potassium oxide has lower pH

Platinum acts as a heterogeneous catalyst in the reaction between ammonia and oxygen. It provides an alternative reaction route with a lower activation energy. Describe the stages of this alternative route. (3 marks)

• Reactant(s) adsorbed onto the (platinum surface) / (platinum) provides a surface / active sites

• Reaction (on the surface) or bond breaking(weakening) / bond making occurs (on the surface)

• Desorption (of the product)